Results in Engineering (Mar 2023)
Numerical and experimental investigation of surface-stabilized combustion in a gas-fired condensing boiler
Abstract
Gas-fired condensing boilers are widely used in domestic heating. However, emission of nitrogen oxides (NOx) remains a concern due to their adverse environmental effects. Employing lean premixed surface-stabilized combustion, ultra-low NOx generation is achievable.We studied NOx formation in realistic, axi-symmetric condensing boiler setups featuring lean combustion of methane stabilized by a porous cylindrical burner using a dedicated numerical model. To accurately represent the physical conditions, our model comprises most notably laminar combustion using a GRI-3.0 kinetic mechanism, a volumetric heat transfer coefficient for heat exchange inside the porous solid, conjugate heat transfer at the gas-solid interface and radiative heat exchange between burner surface and combustion chamber wall. A comprehensive assessment included validation of individual model aspects against literature data, a mesh study and validation with experimental measurements from a full-scale condensing boiler setup. Our findings suggest that the effective diffusion due to dispersive flow in porous media is critical for accurately capturing the reaction zone, thus preventing overprediction of flame temperature and NO concentration.The dependencies of NOx generation and heat release parameters on varying parametric and geometric configurations of the setup were assessed in a comprehensive numerical parameter study. We found that NO emission increases in a near-linear way with firing rate, while radiation efficiency decreases. At constant input power, NO emission rises with increasing equivalence ratio alongside radiation efficiency and peak flame temperature. By optimizing inflow uniformity, peak temperatures can be reduced by 9%, leading to significantly lower NOx generation without affecting the heat flow rate.
